I like it…I like it a lot. It lets fist-using monks catch up with their kama brethren in the lower-magic worlds in terms of dmg and the Hp is pretty damn good too. If it also has fist damage progression, like sacred fist, that would really make it worthwhile, otherwise, it's somewhat better, but I'm not sure I would take it. I really like the stacking with Diamond Soul for SR, that's something I haven't seen before, and it's one of the classes' real strength. I really wish SF had it too, but hey, what can you do. Still probably would take it though if I played a fist-monk. Yeah Loki & Kunikos, we all wish to modify flurry to allow short-swords, daggers and all that, as it should be, but we have to beg Obsidian for that.

A 20 monk has 2d10 fist damage. A 30 monk has 3d10 fist damage. This class provides +10 elemental damage (which bypasses DR). So it's twice as strong as normal monk progression damage wise (on average). The AC bonus also keeps it at the same level as a pure monk. So you get a +2 AB bonus, +elemental damage, no loss of SR, some minor elemental resistance, and the ability to turn stunning fist in a small AE blast.

It isn't meant to compete with the power level of a Sacred Fist, not by a long shot.

I think your addition of this Elemental Ki Strike is a great way to get around the lack of unarmed damage progression and still give the Monk some much needed flavor.

One question regarding your proposed implementation of Elemental Soul: is there a particular consideration for it showing up in the latter half of the Elemental Monk progression as opposed to earlier? I noticed that the qualification for Elemental Monk as described now requires at least Monk 7 for the Wholeness of Body feat and another +2 base attack bonus worth of levels to achieve so the earliest a player could gain the benefit of Elemental Soul would be character level 17 or 18. Not that that's necessarily a bad thing but it seemed significant enough to bring it up.